Disposable absorbent products, and particularly disposable absorbent bandages, diapers and sanitary napkins are well known. An object common to such products is the provision of an effective means for absorbing aqueous liquids such as urine deposited thereon in such a manner as to prevent run-off during maximum flow conditions and to effectively retain such liquids absorbed within the absorbent core member of the structure. It is likewise an object of such structures to reduce the amount of moisture in contact with the wearer's skin and thus reduce maceration, rash or other unpleasantness. Such structures should preferably exhibit both strength and softness to enhance their absorbency and containment characteristics in addition to wearer comfort. Prior art disposable absorbent products have not, however, successfully combined all of the aforementioned desirable features in a single structure.
Structures comprised of a porous hydrophobic topsheet and a hydrophilic substrate are well known in the art. It should be recognized that the terms "hydrophobic" and hydrophilic as herein employed, while useful in their brevity actually refer, respectively to relatively low and relatively high critical surface tensions of the materials being characterized. The hydrophobic nature of a prior art diaper topsheet, for example, is evidenced by its lack of affinity for liquid human waste relative to that of the hydrophilic or absorbent substrate. As used herein, a web is hydrophobic when a drop of liquid waste placed thereon does not spread to any appreciable degree on the web. Thus, when a hydrophobic sheet is superimposed upon a layer of hydrophilic or less hydrophobic absorbent material to form a diaper and the hydrophobic material is placed next to wearer's skin, waste fluids from the wearer pass through the hydrophobic sheet and are preferentially partitioned by and absorbed within the underlying hydrophilic layer, leaving the topsheet adjacent the wearer's skin relatively dry.
The rate at which liquid penetration takes place varies greatly, however, depending upon such factors as the density and capillary gradients existing between the various layers of the absorbent structure and the relative hydrophobicity of the various layers. Capillary forces act to draw a liquid from a less dense structure to a more dense hydrophilic structure, i.e., from a large pore size to a smaller pore size, and this directional action can be augmented further by employing a topsheet which is both lower in density and slightly hydrophobic with respect to the hydrophilic absorbent core to promote rapid absorption of the liquid deposited on the topsheet by the absorbent core member, thereby minimizing liquid run-off during periods of heavy flow. Care must be taken, however, that the surface dryness of the structure not be adversely affected. If an extremely thin, low density, hydrophobic topsheet is emoloyed in combination with a more dense hydrophilic core, fluids absorbed by the hydrophilic core may not be effectively prevented from re-emerging at the surface of the topsheet when the structure is subjected to compressive forces generated by the activities of the wearer.
In general, a more hydrophobic topsheet provides poorer strike-through characteristics but better surface dryness, while a less hydrophobic topsheet provides better strike-through characteristics but poorer surface dryness. Thus to a certain extent there has been a balancing in prior art structures between favorable strike-through characteristics and favorable surface dryness characteristics.
As used hereinafter, strike-through is a measure of how long it takes for an absorbent structure to completely absorb a specified quantity of liquid deposited on its surface. Strike-through values are normally expressed in seconds. Low strike-through values are indicative of high absorbency rates, and are, therefore, generally preferred in absorbent bandages and the like. Surface wetness, on the other hand, is a measure of the degree to which absorbed moisture can be caused to re-emerge from an absorbing matrix, under pressure, to appear as moisture at the surface of the structure where it originally entered the absorbing matrix. Surface wetness values are normally expressed in grams of re-emerging liquid absorbed on a standard filter paper superposed on the absorbing matrix. Low surface wetness values are indicative of a greater ability of the absorbent structure to retain an aqueous solution once it has been absorbed, i.e., improved surface dryness.
One prior art means of providing satisfactory surface dryness characteristics while minimizing liquid run-off is disclosed in U.S. Pat. Re. No. 26,151 which issued to Duncan et al. on Jan. 31, 1967. The Duncan et al. patent discloses a disposable diaper structure employing an absorbent pad and a waterproof backsheet of a width greater than that of the pad. The side portions of the backsheet are folded inwardly over the side marginal areas of the pad so that in use liquid runoff is minimized during periods of heavy flow. The inwardly folded side portions assume a position contiguous to the wearer's legs along an area of the inner, rear and front portions of the thighs adjacent the junction thereof with the wearer's torso, thus providing a gasketing action. In the described embodiment, a hydrophobic topsheet encloses the absorbent material of the pad. The gasketing action gives the absorbent pad sufficient time to absorb liquid wastes to thus utilize, as fully as possible, the absorptive power of the diaper while simultaneously preventing run-off during periods of heavy flow and consequent soiling of garments.
While side flaps have proven to be one acceptable solution to the run-off problem in a disposable diaper exhibiting satisfactory surface dryness characteristics, applicants' invention, in a preferred embodiment, provides rapid strike-through and improved surface dryness characteristics in a single structure, thus making the use of side flaps to prevent liquid run-off unnecessary. Applicants' structure is, therefore, simpler in terms of construction as well as in its application to the wearer.
Another problem common to most prior art disposable absorbent structures relates to a lack of strength in the absorbent core materials employed. This problem is particularly apparent with airfelt, an air laid cellulosic material which is widely used in disposable absorbent products due to its desirable absorbency and softness characteristics and its relatively low cost. Disposable absorbent products, and particularly disposable diapers fabricated with absorbent cores of unreinforced airfelt do not, however, have sufficient strength to resist tearing and shredding in use. Tearing and shredding of the absorbent core in a disposable diaper is undesirable in that it adversely affects both the absorbency and containment characteristics of the structure in addition to being aesthetically unacceptable to the consumer. Prior art disposable absorbent products have dealt with this problem by various means, including, for example, wet calendering of the airfelt, wet-strength tissue addition, embossing of the airfelt, addition of adhesive strips to the airfelt, etc.
U.S. Pat. No. 3,612,055 which issued to Mesek et al. on Oct. 12, 1971 discloses a disposable diaper having an absorbent core of loosely compacted cellulosic batt having greater wettability than that of the facing web, said batt having a highly compacted layer on its back side which is adhered directly to a waterproof backing sheet over a widely distributed area of adhesion. The densified surface is produced by calendering the absorbent web while the surface to be densified is in a moist condition. Liquid deposited on the surface of the structure disclosed by Mesek et al. passes through the body of the loosley compacted batt and is strongly drawn into the densified layer due to the small capillary radius of the of the densified fibers. Such a diaper, although effective from a liquid absorption and liquid retention standpoint, has a rather stiff feel due to the fact that the densified layer of the absorbent core is in adherent contact with the waterproof backsheet, thereby tending to impart the stiffness of the absorbent core to the entire structure.
U.S. Pat. No. 3,444,859 which issued to Kalwaites on May 20, 1969 discloses alternative means for reinforcing a fibrous batt to impart mechanical strength thereto by foam bonding the exterior surfaces of the batt to form a relatively strong skin thereon. The skin on the exterior surfaces of the batt, however, detracts from the softness and texture of a disposable diaper structure incorporating the batt as an absorbent core.